the effects of cialis may be more predictable if you have a stable level of it in your body after repeated, regular dosing.
but it actually can be used on a "when necessary" basis too. well at least thats what their website says:
"
†In clinical trials, 36-hour CIALIS started to work in 30 minutes for some men who took 20 mg of 36-hour CIALIS. When taken as needed, 36-hour CIALIS was shown to improve, up to 36 hours after dosing, the ability of men with ED to have a single successful intercourse attempt."
Erectile Dysfunction (ED) Treatment with 36-Hour CIALIS (tadalafil) tablets